1. Field of the Invention
The present invention relates to an ultrasonic diagnostic equipment wherein color Doppler image data are generated on the basis of ultrasonic Doppler signals obtained from a patient.
2. Description of the Related Art
An ultrasonic diagnostic equipment is such that ultrasonic pulses which have been generated from piezoelectric transducers built in an ultrasonic probe are radiated into a patient, and that ultrasonic reflected waves generated by the difference of the acoustic impedance of a patient tissue are received by the piezoelectric transducers and then displayed on a monitor. Such a diagnostic method is extensively employed for the functional diagnoses and morphological diagnoses of the various internal organs of a living body because a two-dimensional image can be easily observed in real time by the simple operation of merely bringing the ultrasonic probe into touch with the surface of the body.
An ultrasonic diagnostic method which obtains in-vivo information on the basis of reflected waves from a tissue or blood corpuscles within a living body has made rapid progress owing to the great technological developments of two methods; an ultrasonic pulse echo method and an ultrasonic Doppler method. A B-mode image and a color Doppler image which are obtained using the technologies, are indispensable to ultrasonic image diagnoses of today.
A color Doppler method is such that, in a case where a predetermined section within a living body has been scanned with ultrasonic pulses and where a moving reflector such as blood (blood corpuscles) has been irradiated with ultrasounds, imaging is performed by grasping a Doppler frequency shift which is induced in correspondence with the velocity of the reflector (blood flow velocity). The color Doppler method was initially employed for the imaging of a blood flow within the cardiac cavity as exhibits a high blood flow velocity, but nowadays it has become applicable even to the imaging of a very slow blood flow such as the tissue blood flow of an abdominal organ.
In order to enhance a diagnosability with the color Doppler method, a superior measurement precision (low-flow-velocity detectability and high-flow-velocity detectability) is first required, a temporal resolution (real-time responsivity) is secondly done, and a spatial resolution is thirdly done.
Now, the measurement precision being the first requisite will be explained. In a case where a moving reflector is irradiated with ultrasonic pulses and where the moving velocity of the reflector is measured from the Doppler frequency shift of reflected waves from the reflector, it has heretofore been practiced that ultrasound transmissions/receptions to and from the reflector are repeated a plurality of times (L times) at predetermined transmission/reception intervals Tr, and that the moving velocity is measured on the basis of a series of reflected waves obtained within an observation time Tobs (Tobs=Tr·L)
In this case, the detectability for the reflector of low flow velocity (low-flow-velocity detectability: the lower-limit value of measurable flow velocities) as denoted by “Vmin” is determined by the characteristics of a filter, namely, the cutoff frequency and roll-off characteristic of the filter (for example, MTI filter) which is used for detecting Doppler components from within the series of reflected waves obtained by the L times of ultrasonic transmissions/receptions (hereinbelow, simply termed “transmissions/receptions”). The detectability Vmin on this occasion is indicated by the following equation (1) where “fr” (fr=1/Tr) denotes a transmission/reception repetition frequency (rate frequency: pulse repetition frequency (PRF)):
                                                        V              ⁢                                                          ⁢              min                        ⁢                                                  ∝                          1              Tobs                                =                      fr            L                          ⁢                                                      (        1        )            
On the other hand, the upper-limit value of the measurable flow velocities (high-flow-velocity detectability) as denoted by “Vmax” is determined by a Nyquist frequency which is defined by ½ of the transmission/reception repetition frequency (rate frequency) fr, and it is indicated by Equation (2) below. In the equation, “C” denotes an acoustic velocity value within the patient, “fo” denotes the center frequency of received ultrasonic waves, and “ξ” denotes an angle which is defined between an ultrasonic transmission/reception direction and a blood flow direction. Besides, in a case where the Doppler frequency shift has exceeded the Nyquist frequency, an aliasing phenomenon appears in the frequency spectrum of Doppler signals, and hence, the precise measurement of the blood flow velocity becomes impossible.
                              V          ⁢                                          ⁢          max                =                              C            ·            fr                                4            ⁢            fo            ⁢                                                  ⁢            cos            ⁢                                                  ⁢            ξ                                              (        2        )            
More specifically, in order to enhance the low-flow-velocity detectability Vmin which is one factor of the first requisite in the color Doppler method, the rate frequency fr needs to be set low, or the number of times L of the repetitive transmissions/receptions in a predetermined direction needs to be increased. On the other hand, in order to enhance the high-flow-velocity detectability Vmax which is the other factor of the first requisite, the rate frequency fr must be set high. Since, however, the rate frequency fr has its upper-limit value determined by the field of view, it cannot be heightened still further.
Meanwhile, the real-time responsivity being the second requisite is determined by the number of display images per unit time (frame frequency) “Fn”. The frame frequency Fn is indicated by Equation (3) below. In the equation, “M” denotes the total number of scan directions which are necessary for the generation of the data of one color Doppler image, and the number of times L of the transmissions/receptions or the total number M of the scan directions must be set small in order to enhance the real-time responsivity.
                    Fn        =                              fr                          L              ·              M                                =                                    1                              Tobs                ·                M                                      ∝                                          V                ⁢                                                                  ⁢                min                            M                                                          (        3        )            
Further, in order to enhance the spatial resolution being the third requisite, the total number M of the scan directions needs to be increased. That is, the frame frequency Fn (real-time responsivity), the low-flow-velocity detectability Vmin as well as the high-flow-velocity detectability Vmax, and the spatial resolution are in the trade-off relationship, and they are difficult of being simultaneously satisfied. Therefore, importance has been attached to the frame frequency Fn and the high-flow-velocity detectability Vmax in case of blood flow measurements in the region of circulatory organs, and to the frame frequency Fn and the low-flow-velocity detectability Vmin in case of blood flow measurements in abdominal and peripheral organs.
In order to cope with the above problems, a staggered pulse scheme capable of enhancing the high-flow-velocity detectability has been proposed in, for example, JP-A-4-197249 (hereinbelow, termed “Patent Document 1”).
The staggered pulse method is a technique wherein transmissions/receptions in a predetermined direction are repeated at, for example, two different transmission/reception intervals T1 and T2 (T2=T1+Ts), and a blood flow velocity is calculated on the basis of the difference value Δθ (Δθ=θ2−θ1) between the phase difference θ1 of reception signals obtained by the transmission/reception at the transmission/reception interval T1 and the phase difference θ2 of reception signals obtained by the transmission/reception at the transmission/reception interval (T1+Ts). An aliasing frequency in this method is determined by the difference Ts of the transmission/reception intervals, and the high-flow-velocity detectability can be bettered by setting the difference Ts to be T1>Ts.
On the other hand, for the blood flow measurements of low flow velocities in the abdominal organs, peripheral blood vessels, etc., a new scan method (hereinbelow, termed “interleave scan method”) has been proposed in, for example, JP-A-64-43237 (hereinbelow, termed “Patent Document 2”). FIG. 1 shows a practicable example of the interleave scan method disclosed in Patent Document 2. The upper stage of the figure indicates transmission/reception directions (hereinbelow, termed “scan directions”) θ1 through θM in a sector scan, while the lower stage indicates the sequence of transmissions/receptions in the scan directions.
More specifically, with this method, ultrasounds are first transmitted/received in the direction of the scan direction θ1 at a time t1. Subsequently, ultrasounds are transmitted/received in the direction of the scan direction θ2 at a time t2, and in the direction of the scan direction θ3 at a time t3. Further, the transmissions/receptions of ultrasounds are repeated in the Q (Q=3) directions of the scan directions θ1-θ3 again at times t4-t6 and times t7-t9. When the L times (L=3) of transmissions/receptions have been respectively completed at intervals Ts (Ts=3 Tr) in all the directions of the scan directions θ1-θ3 in this way, L times of transmissions/receptions based on the intervals Ts are similarly performed in the directions of the scan directions θ4-θ6, the scan directions θ7-θ9, . . . . According to this method, premising that a limit down to which the cutoff frequency can be lowered with a predetermined attenuation factor satisfied by an identical design method depends upon the observation time “Tobs”, the low-flow-velocity detectability “Vmin” becomes the following equation (4).
                                          V            ⁢                                                  ⁢            min                    ∝                      1                          Tobs              ·              Q                                      =                              fr                          Q              ·              L                                =                                    fs              L                        ⁢                                                  ⁢                          (                              Q                =                3                            )                                                          (        4        )            
Here, “fs” (fs=1/Ts) denotes a transmission/reception repetition frequency for the respective scan directions. That is, according to the practicable example, the transmission/reception repetition frequency fs becomes ⅓ of the rate frequency fr in the case where the interleave scan is not performed. It is accordingly permitted to enhance the low-flow-velocity detectability triple without lowering the frame frequency.
The proposed methods, however, are problematic as stated below. According to the method of Patent Document 1, the phase differences θ1 and θ2 in the Doppler signals obtained by the transmissions/receptions at the different transmission/reception intervals T1 and T2 are respectively evaluated, and the blood flow information is further calculated on the basis of the difference between the phase differences θ1 and θ2. Therefore, the blood flow information to be obtained is liable to become unstable under the influence of noise. In particular, a measurement error which is not negligible develops due to speckle noise ascribable to the interference of ultrasounds.
Further, with this method, the transmissions/receptions at the transmission/reception intervals T1 and T2 are repeated, so that the filter (MTI filter) for extracting the Doppler signals has a blind frequency which is determined by, for example, the sum between the transmission/reception intervals T1 and T2. Accordingly, a Doppler component agreeing with the blind frequency lowers conspicuously to make impossible the calculation of the blood flow information at a high precision.
On the other hand, the method of Patent Document 2 has been proposed for the purpose of measuring the comparatively slow blood flows of the abdomen, etc. Although the low-flow-velocity detectability Vmin is enhanced to 1/Q, also the high-flow-velocity detectability Vmax becomes 1/Q, and the frequence of occurrence of the aliasing phenomenon heightens for fast blood flows. It is therefore impossible to apply this method to the measurements of comparatively fast blood flows.
Concretely, as shown in FIG. 2A, when the region of interest is scanned with a small number of data, for example, 8, the blood flow of low flow velocity is cut by a wall filter and is not displayed by the prior-art method (interleave scan of one stage) in a case where the wall filter has characteristics as shown in the figure.
In contrast, with the method of the interleave scan (of, for example, 2 stages) as disclosed in Patent Document 2, even when the region of interest is scanned with the same number of data, 8 data, the aliasing frequency becomes half. As shown in FIG. 2B, therefore, the characteristics of the wall filter improve, and the blood flow being invisible with the prior-art method becomes visible. However, the problem occurs that a blood flow which goes away is displayed like a blood flow which turns back and comes near.
As shown in FIG. 2C, this problem is solved when the region of interest is scanned with the double number of data, 16 data, by the prior-art method (interleave scan of one stage) At this time, however, the other problem occurs that the frame rate (temporal resolution) lowers.